The objectives of this proposal are to examine the structure, organization and evolution of two multicopy DNA families displaying mobility within mammalian genomes in order to approach an understanding of the molecular nature of the mechanisms that result in evolutionary changes with mammalian genomes such as transpositions and chromosomal alterations. Multicopy DNA families such as ribosomal genes and tandemly repetitious satallite DNAs (hrDNAs) exhibit mobility within mammalian karyotypes and have been shown to participate in chromosomal rearrangements. The most frequent chromosomal rearrangements in humans not associated with disease states involve chromosomes containing ribosomal gene loci. Chromosomal fusions in humans have also been demonstrated to involve breaks in hrDNA sequences. Specific aims include the examination of intra- and interspecific variation in restriction maps, nucleotide sequences and methylation states of cytosine residues in equid hrDNAs. Notable for the highest estimated rate of chromosomal evolution among mammals, the genus Equus displays wide variation in diploid chromosome number, in the location of nucleolus organizer regions, and a multiplicity of buoyant density hrDNAs. A comparison of the DNA sequences of centromerically localized hrDNA of the Mongolian wild horse and the centromerically and telomerically localized hrDNA of Hartmann's mountain zebra will be completed. Junctions between hrDNA and non-repetitive DNA will be cloned in plasmid vectors and analyzed for differences between hrDNA sequences at the ends of arrays and the consensus sequence. Patterns of nucleotide substitution in hrDNA and ribosomal genes will be scrutinized for evidence of unequal genetic exchange and conversion events. The molecular changes that occur in the chromosomal DNA of higher organisms and the mechanisms that produce them are of fundamental significance because they are the basis of phenotypic variation, chromosomal alterations, chromosomal evolution and speciation. Intra- and interspecific comparisons of multicopy DNAs will be conducted to gain insights into the rates and modes of evolution of these hrDNAs and the coding, non-coding and flanking regions of ribosomal gene loci. Time scales of the rates of change of multicopy DNA will be compared with known speciation events in order to compare the predictions of alternative theories suggesting a causal role for hrDNA sequences in speciation (genomic drive, genome resetting, genomic disease, etc.).